On Thu, Dec 01, 2016 at 06:52:35AM +0100, Peter Zijlstra wrote:
> On Wed, Nov 30, 2016 at 01:13:03PM -0600, Josh Poimboeuf wrote:
> > This question was probably intended for other folks, but I should point
> > out that idle tasks *do* invoke the scheduler. cpu_idle_loop() calls
> > schedule_preempt_disabled().
>
> Right, but that doesn't matter I think. The below will simply not call
> rcu_note_voluntary_context_switch() from the idle task, which would be
> fine I think.
>
> > > So is the following a sensible approach, or should I look elsewhere?
> > >
> > > #define cond_resched_rcu_qs() \
> > > do { \
> > > if (!is_idle_task(current) && !cond_resched()) \
> > > rcu_note_voluntary_context_switch(current); \
>
> You should reverse your conditions though:
>
> if (!cond_resched() && !is_idle_task(current))
> rcu_note_voluntary_context_switch(current);
>
> That way we'll still do cond_resched() and you only gate the RCU call.
This makes it illegal at early boot. This is not a problem with the
surviving cond_resched_rcu_qs(), but one of the candidates really was
called at boot time. If I reverse the order as you say, I can just as
well leave of the "!is_idle_task(current)".
So I will just drop this patch until such time as someone actually needs
to add a cond_resched_rcu_qs() that sometimes gets invoked at boot time.
Thanx, Paul
